In conventional mechanical structures, parts are often connected together using flanges and counter-flanges. As a general rule, a flange is a portion extending at an angle from the end of a part along all or part of its circumference, and used for joining said part to another part by bolting, the bolts passing through the flange. A counter-flange is a reinforcing part that presents the same general shape as the flange and that is disposed between the heads of the bolts and the flange. The function of the counter-flange is to distribute the forces that hold the part over the entire flange. Without a counter-flange, those forces would be concentrated on the flange in the vicinity of the heads of the bolts, thereby weakening the flange. In order to perform its function correctly, the counter-flange must be a good fit to the surface of the flange.
When the part is made of composite material (such a part generally being referred to as a “composite part”), the flange forming a portion of the part is likewise made out of the same composite material. The counter-flange is generally made out of a different material. More recent composite parts are made from a fibrous structure that has been three-dimensionally woven using fibers (of carbon, kevlar, glass, etc.) and densified with a polymer. Thus, in a composite part, the fibrous structure is embedded within a solid polymer matrix, which matrix is reinforced by the fibrous structure.
By way of example, one known technique for performing such densification is impregnation using a liquid: a distinction is drawn between infusion and injection, and both of those techniques are described elsewhere.
Thus, starting with a composite part fabricated using the above techniques, in order to ensure that the flange and the counter-flange fit together well, various solutions are possible.
The solution that consists in machining the regions of the flange that come into contact with the counter-flange so that the flange is a good fit with the surface of the counter-flange cannot be accepted since such machining would cut the fibers of the fibrous structure, thereby compromising the mechanical integrity of the flange, since it is the fibers that provide its mechanical strength.
The solution that has been used in the past consists in fabricating a counter-flange out of metal, e.g. titanium, that is a good fit with the surface of the composite flange, the flanges and the counter-flanges being fabricated as separate series. It should be observed that each composite part is unique and, in particular, it presents an outside surface that often differs from other parts fabricated using the same mold and the same technique. These variations from one composite part to another run the risk of creating contact defects between the flange and the counter-flange, and consequently of giving rise to subsequent damage. In addition, potential point contacts between the fibers of the flange and the metal counter-flange run the risk of giving rise to subsequent damage. Similarly, any residual zones forming pockets of resin between the flange and the counter-flange also run the risk of giving rise to subsequent damage. Furthermore, metal is a material that is heavy compared with composite materials, having about three times the density, thereby also presenting a drawback in the context of optimizing the weight of turbojets and turbine engines.